Hi
galaxy flyer, hawk37, and willl05,
Assuming the generators and hydraulics are operational, there is power on a stopped jet. Small point
See my argument below.
Heat energy, and kinetic energy (exhaust) come to mind.
Power plants use turbines to produce electrical power, however of course, like the stopped aircraft, the power plants are not moving.
I'm sure the quote/myth is really meant to mean "there is no power (ie energy) gained by the aircraft" (except possibly warmer brakes).
However I've seen it referred to as "the aircraft is producing no power", clearly incorrect.
I don't think you can 'gain power' in a physical sense. You can gain energy and convert that energy into work. The rate of doing that work is a measure of the power.
If the myth had stated, 'there is no
energy in a stationary aircraft' I would absolutely agree that it was nonsense. There is a massive amount of energy, just waiting to be converted into work and power!
If you were to stand behind a running engine, you could test if really no work is produced.
I would say that work is being done on the air, not on the aircraft, as it is the air that is being accelerated, not the aircraft.
An interesting set of responses. It all comes down to semantics I suppose.
I was concentrating purely on the performance aspects of the whole aircraft, i.e. Power available and Power required. As we know, for all aircraft, power required against speed follows the classic curve that is similar in shape to the total drag curve.
For a jet aircraft, power available against speed is an almost straight line starting at the origin and shooting up and to the right. The faster one goes, the more power is produced (which is one reason that jet aircraft can fly at high TAS).
However, at
zero TAS, there is zero power available; that was my point.
Of course there is considerable
thrust available, as evinced by the exhaust stream. There is also electrical and hydraulic energy available from the engines, accumulators and battery, but I don't see how this relates to the 'myth'. After all, a passenger in the cabin may switch on a torch/flashlight using the chemical energy in that device's battery, but that hardly matters when one is discussing the performance-related power produced by the aircraft.
The aircraft cannot be producing any performance-related power when stationary, as there is no power available, due to the zero TAS. As soon as you release the brakes, the thrust accelerates the aircraft and the power immediately increases, in direct proportion to the speed (assuming constant thrust).
I fully accept that that this is a narrow interpretation of the word 'power' but I thought that this was what the original 'myth' was addressing. Sorry if I got that wrong. Another issue is the casual interchange of the words 'power', 'energy' and 'thrust' in everyday speech. It doesn't cause any difficulties at all, until someone comes up with the statement in the myth! Then some more accurate definitions are required.
We really need someone with a better understanding of physics than mine to put this one to bed!
Eckhard